The present invention relates to a shadow mask type color CRT having a shadow mask.
A shadow mask type color CRT is composed of a fluorescent screen having countless fluorescent stripes arranged in parallel to each other at regular intervals, an electron gun disposed opposite to the fluorescent screen and a shadow mask having countless electron beam holes (hereinunder referred to simply as "holes") disposed with a predetermined positional relationship with the fluorescent stripes. The shadow mask is disposed in the interior of the CRT substantially in parallel to and in proximity to the fluorescent screen.
The current density (namely, stimulation density) in a certain section of the electron beam projected from the electron gun is not always constant and ordinarily has a Gaussian distribution or a density distribution approximate thereto. The electron beam is deflected by a deflection yoke which is attached to the outside of the tube of the color CRT in the vicinity of the gunpoint of the electron gun and then enters the shadow, mask in such a manner as to form scanning lines perpendicular to the fluorescent stripes at regular intervals. A part of the electron beam passes through the holes provided in the shadow mask and enters the fluorescent screen so as to selectively cause the fluorescent stripes to emit light.
Each of the holes provided in the shadow mask has a rectangular shape having a substantially constant length and is divided by a bridge having a substantially constant width in the direction perpendicular to the scanning line. These holes are arranged in alignment substantially in parallel to the fluorescent stripes in correspondence therewith, thereby constituting a hole group for each line. In each of the hole groups, bridges are provided so as to make the relative spaces P.sub.S of the holes substantially constant in the respective lines.
The bridges are provided at the intersections of the axes of the hole groups and a plurality of parallel lines which are perpendicular to the lines of the hole groups. That is, on the shadow mask, a plurality of hole groups are provided in lines and a plurality of parallel lines are provided perpendicularly to the axes of these holes, as described above. These axes and the parallel lines constitute an imaginary lattice on the shadow mask, and the bridges are disposed at the intersections of the imaginary lattice. If the parallel lines are set at a predetermined pitch P.sub.A =P.sub.S /2, a bridge is first disposed at an intersection and subsequent bridges are disposed in series at intersections diagonal to the precedent intersection. In this case, with respect to the adjacent holes, the bridges are disposed at a pitch of P.sub.A.
In this way, all the bridges are regularly arranged on the shadow mask with a periodicity in the axial direction of the holes. In this state, if the scanning lines formed by the electron beam are arranged at regular intervals on the shadow mask, the amount of electron beam which passes each hole of the shadow mask is different in holes. Thus the amounts of light emission of the fluorescent substances corresponding to the respective holes are different from each other. On the whole, interference fringes appear at a pitch which is larger than either of the pitch of the holes P.sub.S in the axial direction and the pitch P.sub.B of the scanning lines. These interference fringes are called a Moire, which sometimes produces extreme nonuniformity in the period or the intensity of light emission, thereby greatly impairing the picture quality. Generation of a Moire will be slightly analytically explained hereinunder.
The distribution of stimulation density on a fluorescent screen having no shadow mask due to the electron beam which enters the fluorescent screen will first be considered. If the point at which the electron beam enters the fluorescent screen is called a stimulation point, the stimulation points linearly move in one direction (this direction will be referred to as "direction of X") due to the scanning of the electron beam, thereby constituting a scanning line. The scanning lines are arranged at regular intervals in the direction (this direction will be referred to as "direction of Y") perpendicular to the direction of X, thereby constituting a field. Repetition of these operations forms a picture on the fluorescent screen.
The stimulation density which is to be generated on the fluorescent screen having no shadow mask due to the scanning of the scanning lines is constant for each value of Y in the direction of X if the electron beam current is constant, but has a periodicity in the direction of Y, as shown in FIG. 9. If this periodicity is assumed to be a stimulation density distribution function T.sub.B (Y), T.sub.B (Y) is represented by the following formula [1] obtained by Fourier series expansion: ##EQU1## wherein P.sub.B represents the space between adjacent scanning lines which are arranged at regular intervals. It is here assumed that when one scanning line is selected and, the center thereof is determined at Y=0 and all the scanning lines have a stimulation density distribution symmetrical with respect to the center lines of the respective extensions. The functions B.sub.0, B.sub.1. . . are constants calculated from the stimulation density of the section of the stationary electron beam or the like.
FIGS. 10A and 10B are schematic views of the periodic mosaic pattern of the fluorescent screen and the light emitting portion of a conventional shadow mask type color CRT.
A light emitting portion 1 shown in FIG. 10A has a substantially rectangular shape in correspondence with a hole of a shadow mask (not shown). In each line, the light emitting portions 1 are divided by non-light-emitting portions 2 which correspond to the bridges of the shadow mask and are arranged at a regular pitch P.sub.S in the direction of Y.
The positional relationship between the light emitting portion 1 and the non-light-emitting portion 2 is determined substantially by the specification of the hole of the shadow mask provided between the fluorescent screen and the electron gun due to the structure of the color CRT as is well known, This positional relationship is then projected on the fluorescent screen in a slightly enlarged state. Therefore, strictly speaking, the specification of the light emitting portion 1 and the non-light-emitting portion 2 is not the specification of the shadow mask as it is, but it has substantially the same meaning. For convenience sake, the discussion on the shadow mask will be replaced by the discussion on the fluorescent screen hereinunder. In other words, the light emitting portion 1 corresponds to the hole of the shadow mask and the non-light-emitting portion 2 corresponds to the bridge between the holes. The pitch P.sub.S corresponds to the space between the bridges arranged in the direction of Y on the shadow mask.
When the electron beam impinges on the shadow mask surface at a constant electron energy (stimulation density), a part thereof passes through the holes to cause the fluorescent screen to emit light. At this time, the luminance at one point of the fluorescent screen (which substantially correlates to the shadow mask transmittance at the point corresponding to the point on the fluorescent screen) is assumed to be the luminous efficiency of that point of the fluorescent screen. The luminous efficiencies at each value of Y are averaged over the width of X which is sufficiently larger than spaces between the lines of the hole groups of the shadow mask.
The thus-obtained average luminous efficiency T.sub.A (Y) is a periodic function of 1/2 the space P.sub.S, namely P.sub.A between the adjacent bridges in the same line. T.sub.A (Y) is represented by the following formula [2] obtained by Fourier expansion: ##EQU2## wherein the Y ordinate is the same as divided in the formula [1].
The average luminance L(Y) of each point of the fluorescent screen which emits light when the fluorescent screen is stimulated by the electron beam is divided as follows. The luminance at a value of Y averaged over the width of X which is sufficiently larger than space between the lines of the hole groups of the shadow mask is assumed to be an average luminance at the value of Y and is represented by L(Y). L(Y) is represented by the product of the stimulation density distribution T.sub.B (Y) on the fluorescent screen provided with no shadow mask, which is represented by the formula [1] and the average luminous efficiency T.sub.A (Y) represented by the formula [2]. That is, ##EQU3##
In the formula [3], the first term represents the average luminance of the fluorescent; the second term the distribution of the light emitting portions of the fluorescent screen, namely, the pattern of the light emission itself depending upon the distribution of the holes of the shadow mask; and the third term represents the pattern of the scanning lines itself.
The fourth term can be converted into the following formula: ##EQU4## The two periodical function terms containing ##EQU5## in the formula [4], which are functions of the space period (pitch) smaller than either of the pattern of the distribution of the holes of the shadow mask and the pattern of the scanning lines, produce no problem. However the two periodical function terms containing ##EQU6## which are periodic functions involving a possibility of becoming a very large pitch, produce a large stripe pattern visual with the naked eye and extending in the direction of X depending upon the pitch and the amplitude, is what is called a Moire, thereby making the screen very indistinct. Although in the formulas [2] and [3], T.sub.A (Y) and L(Y) are explained to be average values in a comparatively large area of X, two lines of hole groups are actually sufficient for considering the average value in a shadow mask mainly hitherto, as is clear from FIG. 9. The same stimulated state is repeated in the direction of X at intervals of two lines. Thus another stripe pattern extending in the direction of X is revealed.
The term producing a Moire in the formula [4] can be converted as follows: ##EQU7## That is, the amplitude of the intensity of light is .alpha..sub.m B.sub.n /2 and the pitch is P.sub.A P.sub.B /(mP.sub.B -nP.sub.A) [7]
This pattern is different depending upon the values of m and n, and when (m, n) is determined, the pattern is also determined. It is now assumed that the Moire corresponding to a certain pair of (m, n) is a Moire of the mode (m, n).
The values .alpha..sub.m and B.sub.n generally matter in the range of m =1 to 5 and n=1 to 5. However, since the values .alpha..sub.m and Bn generally decrease as the values m and n increase, consideration of all combinations of m and n in the range of m+n .ltoreq.6 is sufficient. In other words, a general shadow mask type color CRT must be so designed as to have unobtrusive Moires of these modes.
In order to so design a color CRT as to have an unobtrusive Moire, two important measures may be considered. However they respectively have the following problems.
A first measure is to set the space between the bridges of the shadow mask with respect to the non-light-emitting portions in FIGS. 10A and 10B so as to make the pitch of a Moire as small and inconspicuous as possible. More specifically, the pitch of a Moire is represented by P.sub.A P.sub.B /(mP.sub.B -nP.sub.A), so that the space P.sub.A between the bridges namely, P.sub.S is determined so as not to have a very large value of the Moire pitch in the above-described range of m and n. In other words, it is determined so as to preclude a possibility of holding mP.sub.B =nP.sub.A even approximately.
In an ordinary shadow mask type color CRT, the space P.sub.B between the scanning lines is generally provided as an operational condition. Therefore, P.sub.A, namely, the space between the bridges of the shadow mask, is appropriately selected with respect to the given P.sub.B so that the Moire of any mode does not have a conspicuously large pitch.
This method is described at some length, for example, by A. M. Morrell and others on pp. 50 to 62 of "Color Television Picture Tubes" (Academic Press Inc. New York and London, 1974).
However, there is a limitation in the effectiveness of a method of reducing the pitch of a Moire as much as possible by appropriately selecting the pitch of the bridge in the direction of Y.
For example, strictly speaking, the spaces P.sub.B between the scanning lines of a general color CRT are not constant and vary with a certain range by a slight variation of a controlling state or a supply voltage.
It is also required to make the Moire unobtrusive even when the CRT is used in different systems, for example, of NTSC and PAL in which the number of scanning lines are 525 and 625, respectively. If the P.sub.A is selected so as to reduce the pitch of a Moire of a specific mode to an extent which makes it unobtrusive, the P.sub.A may be disadvantageous for a pitch of a Moire of at least another mode.
Generally, when the pitch P.sub.A of the bridges in the direction of Y is selected, the range of the P.sub.A with respect to the given pitch P.sub.B of the scanning lines is roughly determined. Further, the point at which the pitch of the Moire of a mode which matters when a comparatively large value is set as P.sub.A equals the pitch of the Moire of a mode which matters when a comparatively small value is set as P.sub.A is assumed to be the value of compromise of P.sub.A. This compromise, however, is often incomplete. Therefore, and when the characteristic, in the case in which the space P.sub.B between the scanning lines is changed, taken into consideration, the final characteristic is generally very unsatisfactory.
A second measure for designing a color CRT so as to have an unobtrusive Moire is to reduce om in the formula [5] to a negligibly small value, .alpha..sub.m one of the causes for increasing the amplitude ##EQU8## of the intensity of light of a Moire. .alpha..sub.m in the formula [5]is determined by A.sub.m and A.sub.0m in the formula [2]. Since both A.sub.m and A.sub.0m represent the phase relationship of the arrangement of the holes and the arrangement of the scanning lines on the shadow mask, the substantial problem is common.
In other words, A.sub.m and A.sub.0m represent the size of the m-th higher harmonic in the periodic function which represents the average luminous efficiency in the direction of Y determined by the arrangement of the holes on the shadow mask, namely, in a certain range of the direction of X, and are represented by .alpha..sub.m .
As the means for reducing .alpha..sub.m, some methods have conventionally been known.
For example, as shown in Japanese Patent Publication No. 32596/1973 and Japanese Patent Laid-Open No. 33473/1977, the space between the bridges provided between the holes on the shadow mask is made constant in each line. The deviation P.sub.A from the space between the bridges in the adjacent line is set at a value other than P.sub.S /2 as shown in FIGS. 10A. Further the same pattern is repeated in the direction of X at intervals of two to several lines. The amount of deviation may consist of a plurality of repeating values. According to this method, it is indeed possible to reduce a specific .alpha.m to zero, but patterns slightly different from each other are repeated in the direction of X at intervals of two to several lines of hole groups on the shadow mask, or at least patterns assuming the same state in each two to several lines are repeated in the direction of Y at every pitch PS. In other words, a certain pattern of a certain size is periodically repeated, which are perceived by a visually sensitive person as an offensive to the eye.
Another method is a method of arranging the bridges at random in consideration that the regular arrangement of the bridges on the shadow mask produces a Moire. If the bridges are arranged at random, the formula [2] does not hold, thereby preventing a Moire.
This method is disclosed in, for example, Japanese Patent Laid-Open Nos. 744/1975, 40072/1976 and 107063/1976. However, it is necessary in the random arrangement of bridges that the space between the bridges in one line does not exceed a predetermined. This is because if it is larger than the predetermined value, in other words, one hole of a shadow mask has a larger length than the predetermined value, then is a problem in the strength of the shadow mask.
In addition, if the space between the bridges of the holes on a shadow mask is too small, the picture on the screen at that portion becomes very dark. Further, if the positions of the bridges in the adjacent lines of the hole groups are very close, namely, take very close Y coordinate values the picture on the screen at that portion becomes very dark. Therefore since the bridge density becomes low in the vicinity thereof, a problem in the strength produces on the shadow mask.
Therefore, in the above-described known examples, some restrictions are imposed in the random arrangement of bridges in order to eliminate the above-described defects. As a result the completely random arrangement is impossible. The random arrangement of bridges is generally accompanied by nonuniformity due to noise, namely, an irregular luminance distribution which is visually observed and is generally called pepper and salt, snow or freckle. The random arrangement of bridges with the above-described restriction imposed has a smaller Moire removing effect in proportion to the noise caused by the non-uniform position of the bridges, and consequently has some problems in practical use.